As one example of the display modes of a liquid crystal display device, there is a vertical alignment mode wherein the liquid crystal molecules in a liquid crystal cell are aligned vertically with respect to the substrates thereof under the initial conditions. When no electric voltage is applied, the liquid crystal molecules are aligned vertically with respect to a substrate. Therefore, a black image is obtained if linear polarizers are arranged in a orthogonal relation to one another on a liquid crystal cell.
The optical characteristics in the liquid crystal cell is isotropic in the plane direction, and thus ideal viewing angle compensation is easily achieved. In order to compensate a positive uniaxial anisotropy in the liquid crystal cell thickness direction, an optical element with a negative uniaxial isotropy in the thickness direction is inserted between one or both surface of the liquid crystal cell and the linear polarizers, resulting in extremely excellent black display viewing angle characteristics.
When an electric voltage is applied, the liquid crystal molecules changes in alignment from the direction vertical to the substrate surfaces toward the direction parallel thereto. Thereupon, it is difficult to align uniformly the liquid crystal molecules. The use of a conventional alignment treatment, i.e., rubbing treatment leads to a significant reduction in display quality.
In order to align uniformly the liquid crystal molecules when an electric voltage is applied, there is a proposal wherein the shape of electrodes on the substrate is modified to generate an oblique electric field in the liquid crystal layer. According to this proposal, although a uniform liquid crystal molecule alignment is achieved, nonuniformly aligned regions if viewed microscopically are formed and become dark regions when an electric voltage is applied. Therefore, the liquid crystal display device is decreased in transmissivity.
According to Patent Document 1 below, there is a proposal wherein circular polarizers are replaced for linear polarizers arranged on both sides of an liquid crystal element having a random-aligned liquid crystal layer. The use of circular polarizers each comprising a linear polarizer combined with a ¼ wavelength plate in place of the linear polarizers can eliminate dark regions formed when an electric voltage is applied and accomplish the production of a liquid crystal display device with a higher transmissivity. However, a vertical alignment type liquid crystal display device with circular polarizers has a problem that the viewing angle characteristics are narrower than a vertical alignment type liquid crystal display device with linear polarizers. According to Patent Document 2 below, it is proposed to use an optical anisotropic element with a negative uniaxial anisotropy or a biaxial optical anisotropic material in order to compensate the viewing angle of the vertical alignment type liquid crystal display device with circular polarizers. However, the optical anisotropic element with a negative uniaxial anisotropy can compensate the positive uniaxial optical anisotropy in the liquid crystal cell thickness direction but can not compensate the viewing angle characteristics of a ¼ wavelength plate, resulting in a failure to obtain sufficient viewing angle characteristics. Further, upon the production of the biaxial optical anisotropic material, NZ defined as Nz=(nx−nz)/(nx−ny) is −1.0<Nz<0.1 wherein nx and ny indicate the main refractive indices in the plane of the resulting optical anisotropic plate, nz indicate the refractive index in the thickness direction, and nx>ny. Therefore, there is a limit in stretching in the thickness direction and thus the retardation in the thickness direction can not be controlled within a wide rage. Further, since in the foregoing production method, an elongate film is stretched in the thickness direction by utilizing the heat-contraction of a heat contractive film, the resulting retardation plate becomes thicker than the elongate film. The thickness of the retardation film produced by the method is from 50 to 100 μm and is insufficient for low profiling required in liquid crystal display devices or the like.
In order to compensate the viewing angle of a vertical alignment type liquid crystal display device with circular polarizers, Patent Documents 3 and 4 below proposes a structure wherein three types of elements such as an optical anisotropic element with a negative uniaxial optical anisotropy for compensating the liquid crystal cell, a compensation layer that is large in refractive index in the thickness direction for compensating the viewing angle of the ¼ wavelength plate and a polarizer compensating film are combined. However, when each of these 3 types of the films are arranged on both sides of the display device, 6 sheets of the films in total are used and further λ/4 plates are used on both sides, resulting in the use of 8 sheets of these films. Therefore, they can significantly improve the viewing angle but are not practical in view of production cost and thickness.                (1) Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2002-40428        (2) Patent Document 2: Japanese Patent Application Laid-Open Publication No. 2003-207782        (3) Patent Document 3: Japanese Patent Application Laid-Open Publication No. 2002-55342        (4) Patent Document 4: Japanese Patent Application Laid-Open Publication No. 2006-85203        